1. Field of the Invention
The present invention is in the field of radio receivers. More particularly, the present invention relates to a radio receiver, which may desirably take the form of a scanner-type of receiver, having wide-band, full-spectrum receiving capability (nominal receiving frequency from about 10 kHz to about 2600 MHz--with receiving frequency steps as small as 1 Hz). Still more particularly, the present invention relates to a triple-hetrodyne radio receiver which according to the present invention may receive radio signals in a variety of formats or modes, such as in amplitude modulation (AM), Narrow-band frequency modulation (NBFM), wide-band FM (WBFM), single side band (SSB) in both of the upper and lower side bands (i.e., USB and LSB), carrier wave (CW, sometimes referred to as Morris Code format), Citizen's Band (CB), VHF, UHF (i.e., television and other broadcast portions of WBFM), police, commercial, aviation, marine, and other frequency bands, as well as in other formats or modes, including digital formats (dependent upon output-stage demodulating and decoding facilities which may be provided along with the front-end radio receiver).
2. Related Technology
Conventional radio receivers, generally termed "broad-band" or "wide-band" radio receivers are known. Some of these radio receivers take the form of "scanner" type receivers in which the receiver continuously "scans" preselected frequencies one at a time seeking signals to receive. When a scanner detects a signal in one of the selected frequencies, it stops scanning for a while and receives the detected signal. Other conventional radio receivers require the operator to tune the receiver to a frequency to be received.
Some of these radio receivers have a first stage or "front end" radio frequency demodulation stage which includes a number of parallel narrow-band receivers, each configured to cover a comparatively narrow band of the radio frequency spectrum. The RF band coverage provided by these narrow-band receivers is of adjacent frequency bands so that substantially full coverage of the RF spectrum is provided by the plural receivers in combination. In order to cover the RF spectrum, the narrow-band receivers are switched on and off, and in and out of the circuits of the receiver as the user tunes the radio up and down the RF spectrum. In this configuration of radio, each parallel narrow-band front end section will have its own first IF stage, first LO, second IF, and second LO. Additionally, the radio includes the necessary switching apparatus and circuits to enable the narrow-band front end sections one at a time while the others are disabled according to the portion of the RF spectrum being tuned at a particular time. As a result of this multiplicity of parallel front end stages, controls and switching apparatus, the radios of this type are expensive and complex.
An alternative configuration of broad band radio receiver attempts to choose the first IF and first LO, the second IF and second LO, so that internally-generated frequency interferences are outside of the frequencies of interest, so much as is possible. However, this approach results in frequency exclusions, as is further explained. That is, it is known that because of spurs (i.e., spurious responses), inter-modulation distortion, and image frequency responses, certain frequencies of the electromagnetic spectrum are conventionally unavailable for reception by conventional broad-band or wide-band radio receivers of this type.
To recap the conventional understanding of the limitations of wide or broad band radio receivers of this type, it will be recalled that in hetrodyne demodulation of radio-frequency (RF) signals, two signals [the RF signal and a local oscillator (LO) signal] are combined in multiplication. Multiplication of two sine wave signals together provides one cosine signal which is the sum of the two frequencies, and another cosine signal which is the difference of the two frequencies. Usually, the difference signal is of interest for further demodulation processing, and is referred to as an intermediate frequency (IF) signal.
To create the sum and difference signals true multipliers are not normally used. Instead, fast-switching diodes driven by a square-wave LO signal and a balun transformer can be used to achieve the desired multiplication effect. A variety of alternatives are known to the use of fast-switching diodes. For example, balanced mixers, double balanced mixers, double-double balanced mixers, and Gilbert cells (these a true multiplier) are known. Each of these conventional RF demodulation expedients has its advantages and limitations, as are conventionally known. For example, the Gilbert cell does not usually work at high RF frequencies, as will be further considered below.
A spur (spurious response) is created by mixing harmonics of an RF signal and a harmonic of the LO frequency to generate the IF frequency.
An inter-modulation product results from the mixing of two RF signals and their harmonics to produce the IF frequency. Inter-modulation distortion is a type of interference that results from the mixing of integer multiples of signal frequencies in a nonlinear stage or device. The resulting mixing of signal products can interfere with desired signals on the mixed frequencies.
An image occurs when an IF frequency is less than half of the tunable band of a receiver, and the receiver is tuned to an RF signal near the bottom of the tuning range (with a high-side LO). The receiver will detect a signal at a second frequency which equals the desired frequency plus 2 times the IF frequency.
In order to attempt to avoid the limitations imposed by these spurs, inter-modulation products, and images, some conventional radios of the broad or wide band type use "pre-selectors". A pre-selector is a circuit which provides an amplification of a desired frequency signal with attenuation of other frequencies. Usually, a pre-selector is voltage-tunable, and passes a desired frequency with normal gain, while attenuating offending frequencies.
However, the conventional solutions to the problems of receiving the full radio-frequency electromagnetic spectrum are imperfect. As a result, conventional radio receivers of this type are accompanied by frequency range charts informing their users of frequencies or ranges of frequencies at which reception is not possible with acceptable performance of the receiver. As an example, a commercially available example of a conventional broad-band or wide-band radio receiver (which asserts to cover the 500 kHz to 1300 MHz frequency range), includes a chart informing the user that the following frequencies are unavailable for reception:
253-256 MHz, 262-267 MHz, 271-276 MHz, 380-383 MHz, 412-416 MHZ, 531-540 MHz, 556-572 MHz, 624-635 MHz, 810-835 MHz, 860-890 MHz, 915-961 MHz, and 995-1016 MHz PA1 1.59 MHz, 3.18 MHz, 12.58 MHz, 16.78 MHz, 20.97 MHz, 76.8 MHz, 89.6 MHz, 96.0 MHz, 102.4 MHz, 108.8 MHz, 115.2 MHz, 123.58 MHz, 140.34 MHz, 153.6 MHz, 170.36 MHz, 200.38 MHz, 230.4 MHz, 370.74 MHz, 400.78 MHz, 430.78 MHz, and 460.8 MHz PA1 U.S. Pat. No. 3,937,972, issued to E. C. Snell; PA1 U.S. Pat. No. 3,961,261, issued to P. W. Pflasterer; PA1 U.S. Pat. No. 3,987,400, issued to G. H. Fathauer; PA1 U.S. Pat. No. 4,000,468, issued to J. R. Brown; PA1 U.S. Pat. No. 4,027,251, issued to G. H. Fathauer; PA1 U.S. Pat. No. 4,114,103, issued to P. W. Pflasterer; PA1 U.S. Pat. No. 4,123,715, issued to G. H. Fathauer; PA1 U.S. Pat. No. 4,270,217, issued to W. Baker; and PA1 U.S. Pat. No. 4,409,688, issued to W. Baker
As can be seen, although this conventional radio receiver product is termed "continuous coverage", it does not in fact provide continuous reception of the radio frequency electromagnetic spectrum within its operating range.
Another conventional wide-band radio receiver is known in accord with U.S. Pat. No. 4,627,100, issued Dec. 2, 1986, to Shigeru Takano, and assigned to Regency Electronics of Indianapolis, Indiana (hereinafter, "the Takano '100" patent). This wide band radio scanner type of receiver is believed to provide reception from about 25 MHz to about 550 MHz continuously, and to employ upper conversion (i.e., the first LO frequency is higher than the receiving frequency--in the range from 775 MHz to 1300 MHz) so that image response frequencies are outside of the frequency band of interest, so much as is possible. The Takano '100 patent is believed to employ a variable first IF section with a first mixer receiving a first LO frequency from either one or two phase locked loop (PLL) oscillators. A second IF section uses a crystal oscillator to originate the second LO frequency, and includes several multipliers each of different multiplication factor. The multiplier selected provides a second LO frequency (which is a multiple of the crystal oscillator frequency), and which is then mixed at the second IF stage mixer.
In other words, the Takano '100 patent is believed to expand the frequency coverage of the receiver by providing a method of generating a high-frequency stable first LO frequency. However, this approach still results in some frequency exclusions due to the spurs (i.e., internally generated spurious signals).
The following is a list of the frequencies which will not be received or scanned by a commercially available radio receiver according to the Takano '100 patent due to the internally generated interference signals (i.e., the spurs):
Another conventional radio receiver is known from Japanese patent No. H7-10051, issued Feb. 1, 1995, to Yaesu Musen K.K., by inventor Yoshiaki Hashimoto (hereinafter, the "Yaesu" patent). The Yaesu patent is believed to teach a short wave, single-side-band type of receiver, in which a multiple-conversion super-hetrodyne type of front end receiver section is used. The front end receiver section has a first LO selectively controlled by a CPU to convert the received signal to a first intermediate frequency (i.e., to a first IF). A second LO and second IF are used also, with the second LO signal source being switchable between two oscillators. The CPU controls the second LO signal source to switch the necessary second local oscillator output frequency to the associated mixer so that a spur possibly generated in the mixer falls into a band-pass of the second IF filter. A single side band detector, also controlled by the CPU, is switched between USB and LSB oscillators to provide a detection function.
However, dependent upon whether an upper or lower side band is being used, the CPU of the Yaesu patent controls the first LO to a constant frequency. That is, a first LO frequency may be used for lower side band operation, and a different LO frequency will be used for upper side band operation. But, so long as the Yaesu receiver is operated in upper or lower side band mode, the frequency of the first LO is not changed, it is believed. Additionally, because the Yaesu receiver is apparently a SSB type of short: wave radio receiver, the frequency of the received signal would always be below 30 MHz (the short wave band). The teaching of the Yaesu patent would not appear to apply to a wide-band receiver. Moreover, were an attempt made to apply this teaching to a wide-band receiver, for higher frequencies, a very high LO frequency would be required. Such a high LO frequency leads to instabilities of operation.
Additional s;canner-type radio receivers are known from many United States patents, including:
These patents are believed to be directed to scanner type radio receivers, and to address merely the control of the scanning operation of such a radio receiver. They do not appear to address the problem of internally-generated interferences, or of resulting frequencies or frequency bands which cannot be received by a radio receiver.